(a). Field of the Invention
The present invention relates in general to a communication system, and more particularly to an apparatus for estimating and compensating the carrier frequency offset and phase error in a communication system and a method thereof.
(b). Description of the Prior Arts
In recent years, the orthogonal frequency division multiplexing (OFDM) technology is widely applied to high-speed communication systems, such as asymmetric digital subscriber loop (ADSL), IEEE 802.11a/g wireless local area network (WLAN), etc. FIG. 1 is a block diagram of a typical OFDM communication system 100. The transmitter of the OFDM system 100 first distributes the data under transmission into N frequency-domain subchannels (N=2n, n is an integer) via a signal mapping unit 101, and maintains the orthogonality among the signals of each subchannel to prevent inter-carrier interference (ICI). Next, an inverse fast Fourier transform (IFFT) device 102 is used to transform the subchannel signals into time-domain signals, to which a guard interval (GI) is added by a GI adding device 103. Then, each of these time-domain signals is passed through a parallel-to-serial converter (P/S) 104 and a digital-to-analog converter (DAC) 105, modulated by a carrier and then transmitted via a channel 106. The receiver of the OFDM system 100 first performs carrier demodulation on the received time-domain signals and then an analog-to-digital converter (ADC) 107 is used to sample the demodulated signals. Next, the guard interval of the sampled signals is removed by a GI removal unit 109. The result thereof is provided to a serial-to parallel converter (S/P) 110, and then a fast Fourier transform (FFT) device 110 is used for transforming to frequency-domain signals. Last, the receiver compensates these frequency-domain signals by a channel compensator 112 and performs signal demodulation via a signal demapping unit 113 to recover to the original transmitted data.
A set of N-point IFFT output is typically called a symbol. Since the channel impulse response (CIR) is usually not ideal, a received symbol after passing through the channel 106 would impact the reception of subsequent symbols, i.e. inter-symbol interference (ISI). To prevent ISI, an additional guard interval (GI) is added between two OFDM symbols. Two typical ways to implement the guard interval are zero-padding (ZP) and cyclic prefix (CP). In ZP, a string of zero is added as the guard interval and energy efficiency is thus improved. In CP, a latter portion of a symbol is copied and put before the symbol as the guard interval. CP can reduce the ICI resulted from the channel impulse response. Circuits 103 and 109 of FIG. 1 are used to add and remove the guard interval respectively.
When demodulating OFDM symbols, the receiver of the OFDM system 100 needs to transform the received time-domain signals into frequency-domain signals by the FFT device 111 and performs the demodulation within each subchannel respectively. If synchronization error exists in the time-domain signals inputted to the FFT device 111, then additional ICI and phase rotation would be generated in the output frequency-domain signals to damage the orthogonality of the outputted frequency-domain signals. The system performance would thus be degraded. For the OFDM system, the synchronization error resulted from; (1) carrier frequency offset, (2) carrier phase error, (3) sampling frequency offset, and (4) sampling phase error.
In view of this, the present invention provides an apparatus and a method that can track and compensate the carrier frequency offset by using the pilot signal of an OFDM symbol, thereby upgrading the performance of an OFDM system.